A major goal of the this AADCRC program is to define the role of the epithelial cell barrier in the pathogenesis of asthma and allergic disease and develop new preventative strategies. In that context, this project aims to investigate means by which to block progression from atopic dermatitis to asthma (often referred to as the atopic march). Patients with a history of severe atopic dermatitis (AD) exhibit a 8-to-10-fold greater incidence of developing asthma. Our recent observations demonstrated that in mice, epidermal-derived thymic stromal lymphopoietin (TSLP) was secreted by AD skin. Moreover, circulating levels of TSLP were sufficient to sensitize the lung airways to inhaled allergens in animals lacking any AD-like pathology, preexisting inflammation, or previous exposure to the allergen. In a pilot study in infants we uncovered a correlation between serum TSLP and aeroallergens. Based on these observations in mice and humans, we hypothesize the following mechanism for the atopic march. (1) Epidermal defect/injury during early childhood is sensed by an unknown mechanism that initiates production of TSLP in keratinocytes; (2) keratinocytes secrete TSLP into the serum. (3) Subsequently, circulating TSLP facilitates Th2 immune responses by dendritic cells and T-cells towards innocuous allergens (inhaled or introduced epicutaneously); and (4) this exaggerated adaptive Th2 response results in hypersensitivity to aeroallergens and consequent allergic asthma. We further hypothesize that interrupting some of these events in a model organism will lead to strategies for blocking the development of allergic disease and asthma in humans. To achieve this goal, we propose the to (I) examine how epidermal differentiation/barrier formation defects (intrinsic factors) as well as allergen or pathogens (extrinsic factor(s)) drive TSLP overexpression; (II) with the help of Cores B and C ask how TSLP secretion is regulated by human skin and lung cells in vitro and in patients. Next, (III) we will ask if we can blunt the effects of TSLP in the serum with small molecule adjuvants capable of manipulating the immune responses and (IV) analyze the contribution of the skin microbiome to the maintenance of skin barrier, TSLP expression, and airway hyper sensitivity. Finally, (V) we will confirm TSLP as a risk factor for asthma in a birth cohort (URECA) and, in collaboration with project1, compare its role in an RSV bronchiolitis in early life cohort (RBEL). Achievement of these aims will open up novel therapeutic approach to prevent asthma development in AD patients.